A human retinoic acid receptor which belongs to the family of nuclear receptors

A cDNA encoding a protein that binds retinoic acid with high affinity has been cloned. The protein is homologous to the receptors for steroid hormones, thyroid hormones and vitamin D3, and appears to be a retinoic acid-inducible trans-acting enhancer factor, suggesting that the molecular mechanisms of the effect of retinoids (vitamin A) on embryonic development, differentiation and tumour cell growth are similar to those described for other members of this nuclear receptor family.

Identification of a second human retinoic acid receptor

We have previously described a human complementary DNA that encodes a novel protein which is homologous to members of the steroid/thyroid nuclear receptor multigene family. This novel protein (hap for hepatoma) exhibits strong homology with the human retinoic acid receptor (RAR) which has been recently characterized. To test the possibility that the hap protein might also be a retinoid receptor, a chimaeric receptor was created by replacing the putative DNA binding domain of hap with that of the human oestrogen receptor (ER). The resulting hap-ER chimaera was then tested for its ability to trans-activate an oestrogen-responsive reporter gene (vit-tk-CAT) in the presence of possible receptor ligands. Here we show that retinoic acid (RA) at physiological concentrations is effective in inducing the expression of this reporter gene by the hap-ER chimaeric receptor. This demonstrates the existence of two human retinoic acid receptors designated RAR-alpha and RAR-beta.

Cloning of murine alpha and beta retinoic acid receptors and a novel receptor gamma predominantly expressed in skin

In addition to having profound effects on embryonic pattern formation, retinoic acid (RA) has striking effects on differentiation and maintenance of epithelial cells in vivo and in vitro Skin is a major target organ for retinoids both in its normal and pathological states. The discovery of two human nuclear receptors for RA (hRAR alpha and hRAR beta) acting as transcriptional RA-inducible enhancer factors has provided a basis for understanding how RA controls gene expression. To investigate the specific role that RARs might play during development and in adult tissues, we have cloned the mouse RAR alpha and RAR beta (mRAR alpha and mRAR beta). Their amino-acid sequences are much more homologous to those of hRAR alpha and hRAR beta, respectively, than to each other, which suggests strongly that RAR alpha- and beta-subtypes have different functions. Most interestingly we have discovered a novel RAR subtype (mRAR gamma) whose expression in adult mouse seems to be highly restricted to skin, whereas RAR alpha and RAR beta are expressed in a variety of adult tissues. Furthermore, both mRAR alpha and mRAR gamma RNAs are readily detected in undifferentiated F9 embryocarcinoma (EC) cells, whereas mRAR beta messenger RNA is induced at least 30-fold in RA-differentiated F9 cells.

A third human retinoic acid receptor, hRAR-gamma

Retinoic acid receptors (RARs) are retinoic acid (RA)-inducible enhancer factors belonging to the superfamily of steroid/thyroid nuclear receptors. We have previously characterized two human RAR (hRAR-alpha and hRAR-beta) cDNAs and have recently cloned their murine cognates (mRAR-alpha and mRAR-beta) together with a third RAR (mRAR-gamma) whose RNA was detected predominantly in skin, a well-known target for RA. mRAR-gamma cDNA was used here to clone its human counterpart (hRAR-gamma) from a T47D breast cancer cell cDNA library. Using a transient transfection assay in HeLa cells and a reporter gene harboring a synthetic RA responsive element, we demonstrate that hRAR-gamma cDNA indeed encodes a RA-inducible transcriptional trans-activator. Interestingly, comparisons of the amino acid sequences of all six human and mouse RARs indicate that the interspecies conservation of a given member of the RAR subfamily (either alpha, beta, or gamma) is much higher than the conservation of all three receptors within a given species. These observations indicate that RAR-alpha, -beta, and -gamma may perform specific functions. We show also that hRAR-gamma RNA is the predominant RAR RNA species in human skin, which suggests that hRAR-gamma mediates some of the retinoid effects in this tissue.

The retinoic acid-metabolizing enzyme, CYP26A1, is essential for normal hindbrain patterning, vertebral identity, and development of posterior structures

The active derivative of vitamin A, retinoic acid (RA), is essential for normal embryonic development. The spatio-temporal distribution of embryonic RA results from regulated expression of RA-synthesizing retinaldehyde dehydrogenases and RA-metabolizing cytochrome P450s (CYP26). Excess RA administration or RA deficiency results in a complex spectrum of embryonic abnormalities. As a first step in understanding the developmental function of RA-metabolizing enzymes, we have disrupted the murine Cyp26A1 gene. We report that Cyp26A1-null mutants die during mid-late gestation and show a number of major morphogenetic defects. Spina bifida and truncation of the tail and lumbosacral region (including abnormalities of the kidneys, urogenital tract, and hindgut) are the most conspicuous defects, leading in extreme cases to a sirenomelia ("mermaid tail") phenotype. Cyp26A1 mutants also show posterior transformations of cervical vertebrae and abnormal patterning of the rostral hindbrain, which appears to be partially posteriorly transformed. These defects correlate with two major sites of Cyp26A1 expression in the rostral neural plate and embryonic tail bud. Because all of the Cyp26A1(-/-) abnormalities closely resemble RA teratogenic effects, we postulate that the key function of CYP26A1 is to maintain specific embryonic areas in a RA-depleted state, to protect them against the deleterious effect of ectopic RA signaling.

Differential expression of genes encoding alpha, beta and gamma retinoic acid receptors and CRABP in the developing limbs of the mouse

Retinoic acid has profound effects on vertebrate limb morphogenesis (refs 1-6, reviewed in refs 7-9), including in the mouse, where it can act as a teratogen generating phocomelia and bone defects. A retinoic acid gradient, possibly amplified by a graded distribution of cellular retinoic acid-binding protein (CRABP), could provide positional information across the antero-posterior axis of the chick limb bud. The discovery of nuclear retinoic acid receptors (RARs) acting as retinoic acid-inducible enhancer factors provided a basis for understanding how retinoic acid signals could be transduced at the level of gene expression. We have now used in situ hybridization to study the distribution of messenger RNA transcripts of the three murine receptors (mRARs) and CRABP during mouse limb development. Both mRAR alpha and mRAR gamma transcripts, but not those for mRAR beta, are present and uniformly distributed in the limb bud at day 10 post-coitum, whereas CRABP transcripts have a graded proximo-distal distribution, indicating that differential expression of CRABP, but not of mRAR alpha or mRAR gamma, could participate in the establishment of the morphogenetic field. At later stages, mRAR gamma transcripts become specific to the cartilage cell lineage and to the differentiating skin and mRAR beta transcripts are mostly restricted to the interdigital mesenchyme. CRABP transcripts, however, are excluded from regions expressing mRAR gamma and mRAR beta. These results indicate that all three RARs and CRABP have specific functions during morphogenesis and differentiation of the mouse limb.

cDNA cloning of human retinoic acid-metabolizing enzyme (hP450RAI) identifies a novel family of cytochromes P450

Retinoids, including all-trans-retinoic acid (RA) and its stereoisomer 9-cis-RA play important roles in regulating gene expression, through interactions with nuclear receptors, during embryonic development and in the maintenance of adult epithelial tissues (Chambon, P. (1995) Rec. Prog. Horm. Res. 50, 317-32; Mangelsdorf, D. J., and Evans, R. M. (1995) Cell 83, 841-850; Petkovich, M. (1992) Annu. Rev. Nutr. 12, 443-471). Evidence suggests that 4-hydroxylation of RA inside the target cell limits its biological activity and initiates a degradative process of RA leading to its eventual elimination. However, 18-hydroxylation and glucuronidation may also be important steps in this process. In this paper, we describe the cloning and characterization of the first mammalian retinoic acid-inducible retinoic acid-metabolizing cytochrome P450 (hP450RAI), which belongs to a novel class of cytochromes (CYP26). We demonstrate that hP450RAI is responsible for generation of several hydroxylated forms of RA, including 4-OH-RA, 4-oxo-RA, and 18-OH-RA. We also show that hP450RAI mRNA expression is highly induced by RA in certain human tumor cell lines and further show that RA-inducible RA metabolism may correlate with P450RAI expression. We conclude that this enzyme plays a key role in RA metabolism, functioning in a feedback loop where RA levels are controlled in an autoregulatory manner.

Identification of the retinoic acid-inducible all-trans-retinoic acid 4-hydroxylase

Retinoic acid (RA) metabolites of vitamin A are key regulators of gene expression involved in embryonic development and maintenance of epithelial tissues. The cellular effects of RA are dependent upon the complement of nuclear receptors expressed (RARs and RXRs), which transduce retinoid signals into transcriptional regulation, the presence of cellular retinoid-binding proteins (CRABP and CRBP), which may be involved in RA metabolism, and the activity of RA metabolizing enzymes. We have been using the zebrafish as a model to study these processes. To identify genes regulated by RA during exogenous RA exposure, we utilized mRNA differential display. We describe the isolation and characterization of a cDNA, P450RAI, encoding a novel member of the cytochrome P450 family. mRNA transcripts for P450RAI are expressed normally during gastrulation, and in a defined pattern in epithelial cells of the regenerating caudal fin in response to exogenous RA. In COS-1 cells transfected with the P450RAI cDNA, all-trans-RA is rapidly metabolized to more polar metabolites. We have identified 4-oxo-RA and 4-OH-RA as major metabolic products of this enzyme. P450RAI represents the first enzymatic component of RA metabolism to be isolated and characterized at the molecular level and provides key insight into regulation of retinoid homeostasis.

A retinoic acid-responsive element is present in the 5' flanking region of the laminin B1 gene

The retinoic acid (RA)-associated differentiation of murine F9 teratocarcinoma stem cells results in dramatic changes in gene expression. The cellular gene encoding the B1 subunit of the extracellular matrix protein laminin is transcriptionally activated by RA, and its transcription is further enhanced by N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) during the differentiation of F9 stem cells into extraembryonic parietal endoderm cells. We now report that expression vectors encoding the human RA receptors RAR-alpha, RAR-beta, and RAR-gamma can activate chloramphenicol acetyltransferase (CAT) expression from laminin B1 promoter/CAT expression vectors (e.g., p1.6LAMCAT) in RA-treated F9 cells, as measured in a transient transfection assay. Bt2cAMP does not further enhance the RA-associated increase in CAT activity. Through the use of deletion and mutation analyses, the RA-responsive element (RARE) of the murine laminin B1 gene has been defined as a 46-base-pair element between -477 and -432 of the laminin B1 5' flanking region. Insertion of a region of DNA containing this RARE in either orientation into a thymidine kinase promoter/CAT expression vector causes CAT expression to be activated 5- to 9-fold by the cotransfected human RAR-alpha or RAR-beta constructs in RA-treated F9 cells, and this RARE also functions in human HeLa cells. In contrast, this RARE in the p1.6LAMCAT vector does not activate CAT expression when cotransfected into F9 stem cells with the c-erbA gene in the presence of thyroid hormone. This suggests that the laminin B1 gene is activated by RA but not by thyroid hormone in vivo.

Complementary domains of retinoic acid production and degradation in the early chick embryo

Excess retinoids as well as retinoid deprivation cause abnormal development, suggesting that retinoid homeostasis is critical for proper morphogenesis. RALDH-2 and CYP26, two key enzymes that carry out retinoic acid (RA) synthesis and degradation, respectively, were cloned from the chick and show significant homology with their orthologs in other vertebrates. Expression patterns of RALDH-2 and CYP26 genes were determined in the early chick embryo by in situ hybridization. During gastrulation and neurulation RALDH-2 and CYP26 were expressed in nonoverlapping regions, with RALDH-2 transcripts localized to the presumptive presomitic and lateral plate mesoderm and CYP26 mRNA to the presumptive mid- and forebrain. The two domains of expression were separated by an approximately 300-micrometer-wide gap, encompassing the presumptive hindbrain. In the limb region, a similar spatial segregation of RALDH-2 and CYP26 expression was found at stages 14 and 15. Limb region mesoderm expressed RALDH-2, whereas the overlying limb ectoderm expressed CYP26. RA-synthesizing and -degrading enzymatic activities were measured biochemically in regions expressing RALDH-2 or CYP26. Regions expressing RALDH-2 generated RA efficiently from precursor retinal but degraded RA only inefficiently. Conversely, tissue expressing CYP26 efficiently degraded but did not synthesize RA. Localized regions of RA synthesis and degradation mediated by these two enzymes may therefore provide a mechanism to regulate RA homeostasis spatially in vertebrate embryos.

Apoptotic extinction of germ cells in testes of Cyp26b1 knockout mice

Cyp26b1 encodes a retinoic acid (RA) metabolizing cytochrome P450 enzyme that is expressed in embryonic tissues undergoing morphogenesis, including the testes. We have generated transgenic mice lacking Cyp26b1 and have observed increased RA levels in embryonic testes. Cyp26b1(-/-) germ cells prematurely enter meiosis at embryonic d 13.5 and appear to arrest at pachytene stage. Furthermore, after embryonic d 13.5, a rapid increase in apoptosis is observed in male germ cells derived from Cyp26b1(-/-) embryos; germ cells are essentially absent in mutant male neonates. In contrast, testicular somatic cells appear to develop normally in the absence of Cyp26b1. Moreover, ovarian germ and somatic cells appear unaffected by the lack of CYP26B1. We also show that the synthetic retinoid Am580, which is resistant to CYP26 metabolism, induces meiosis of male germ cells in cultured gonads, suggesting that abnormal development of germ cells in the Cyp26b1(-/-) testes results from excess RA rather than the absence of CYP26B1-generated metabolites of RA. These results provide evidence that CYP26B1 maintains low levels of RA in the developing testes that blocks entry into meiosis and acts as a survival factor to prevent apoptosis of male germ cells.

Identification of the human cytochrome P450, P450RAI-2, which is predominantly expressed in the adult cerebellum and is responsible for all-trans-retinoic acid metabolism

Retinoids, particularly all-trans-retinoic acid (RA), are potent regulators of cell differentiation, cell proliferation, and apoptosis. The role of all-trans-RA during development and in the maintenance of adult tissues has been well established. The control of all-trans-RA levels in cells and tissues is regulated by the balance between its biosynthesis and its catabolism to inactive metabolites. The cytochrome P450 enzyme P450RAI (herein renamed P450RAI-1) is partially responsible for this inactivation of all-trans-RA. In this report, we describe the identification, molecular cloning, and characterization of a second related enzyme, P450RAI-2, which is also involved in the specific inactivation of all-trans-RA. Transiently transfected P450RAI-2 can convert all-trans-RA to more polar metabolites including 4-oxo-, 4-OH-, and 18-OH-all-trans-RA. Competition experiments with other retinoids suggest that all-trans-RA is the preferred substrate. The high level of expression of P450RAI-2, particularly in the cerebellum and pons of human adult brain, suggests a unique role for this enzyme in the protection of specific tissues from exposure to retinoids.

Genetic evidence that oxidative derivatives of retinoic acid are not involved in retinoid signaling during mouse development

Retinoic acid, the active derivative of vitamin A (retinol), is a hormonal signaling molecule that acts in developing and adult tissues. The Cyp26a1 (cytochrome p450, 26) protein metabolizes retinoic acid into more polar hydroxylated and oxidized derivatives. Whether some of these derivatives are biologically active metabolites has been debated. Cyp26a1(-/-) mouse fetuses have lethal morphogenetic phenotypes mimicking those generated by excess retinoic acid administration, indicating that human CYP26A1 may be essential in controlling retinoic acid levels during development. This hypothesis suggests that the Cyp26a1(-/-) phenotype could be rescued under conditions in which embryonic retinoic acid levels are decreased. We show that Cyp26a1(-/-) mice are phenotypically rescued by heterozygous disruption of Aldh1a2 (also known as Raldh2), which encodes a retinaldehyde dehydrogenase responsible for the synthesis of retinoic acid during early embryonic development. Aldh1a2 haploinsufficiency prevents the appearance of spina bifida and rescues the development of posterior structures (sacral/caudal vertebrae, hindgut, urogenital tract), while partly preventing cervical vertebral transformations and hindbrain pattern alterations in Cyp26a1(-/-) mice. Thus, some of these double-mutant mice can reach adulthood. This study is the first report of a mutation acting as a dominant suppressor of a lethal morphogenetic mutation in mammals. We provide genetic evidence that ALDH1A2 and CYP26A1 activities concurrently establish local embryonic retinoic acid levels that must be finely tuned to allow posterior organ development and to prevent spina bifida.

Cloning of a novel retinoic-acid metabolizing cytochrome P450, Cyp26B1, and comparative expression analysis with Cyp26A1 during early murine development

Tight regulation of retinoic acid (RA) distribution in the embryo is critical for normal morphogenesis. The RA-metabolizing enzymes Cyp26A1 and Cyp26B1 are believed to play important roles in protecting certain embryonic tissues from inappropriate RA signaling. We have cloned the murine Cyp26B1 cDNA and compared its expression pattern to that of Cyp26A1 from embryonic day (E) E7-E11.5 using in situ hybridization. Northern blot analysis shows the presence of two Cyp26B1 transcripts of approximately 2.3 and 3.5 kb in embryonic limb bud. Whereas Cyp26A1 is expressed in gastrulating embryos by E7, Cyp26B1 is first expressed at E8.0 in prospective rhombomeres 3 and 5. Cyp26B1 expression expands to specific dorso-ventral locations in rhombomeres 2-6 between E8.5 and E9.5, whereas Cyp26A1 hindbrain expression is limited to rhombomere 2 at E8.5. No (or very weak) Cyp26B1 expression is observed in the tail bud, a major site of Cyp26A1 expression. Differential expression is seen in branchial arches, with Cyp26A1 being mainly expressed in neural crest-derived mesenchyme, and Cyp26B1 in specific ectodermal and endodermal areas. Cyp26B1 is markedly expressed in the ectoderm and distal mesoderm of the limb buds from the beginning of their outgrowth. Cyp26A1 transcripts are seen later and at lower levels in limb ectoderm, and both transcripts are excluded from the apical ectodermal ridge.

A novel human cytochrome P450, CYP26C1, involved in metabolism of 9-cis and all-trans isomers of retinoic acid

Retinoids are potent regulators of cell proliferation, cell differentiation, and morphogenesis and are important therapeutic agents in oncology and dermatology. The gene regulatory activity of endogenous retinoids is effected primarily by retinoic acid isomers (all-trans and 9-cis) that are synthesized from retinaldehyde precursors in a broad range of tissues and act as ligands for nuclear retinoic acid receptors. The catabolism of all-trans-retinoic acid (atRA) is an important mechanism of controlling RA levels in cell and tissues. We have previously identified two cytochrome P450s, P450RAI-1 and P450RAI-2 (herein named CYP26A1 and CYP26B1), which were shown to be responsible for catabolism of atRA both in the embryo and the adult. In this report, we describe the identification, molecular cloning, and substrate characterization of a third member of the CYP26 family, named CYP26C1. Transiently transfected cells expressing CYP26C1 convert atRA to polar water-soluble metabolites similar to those generated by CYP26A1 and -B1. Competition studies with all-trans, 13-cis, and 9-cis isomers of retinoic acid demonstrated that atRA was the preferred substrate for CYP26C1. Although CYP26C1 shares extensive sequence similarity with CYP26A1 and CYP26B1, its catalytic activity appears distinct from those of other CYP26 family members. Specifically, CYP26C1 can also recognize and metabolize 9-cis-RA and is much less sensitive than the other CYP26 family members to the inhibitory effects of ketoconazole. CYP26C1 is not widely expressed in the adult but is inducible by RA in HPK1a, transformed human keratinocyte cell lines. This third CYP26 member may play a specific role in catabolizing both all-trans and 9-cis isomers of RA.

Cytochrome P450RAI(CYP26) promoter: a distinct composite retinoic acid response element underlies the complex regulation of retinoic acid metabolism

The catabolism of retinoic acid (RA) is an essential mechanism for restricting the exposure of specific tissues and cells to RA. We recently reported the identification of a RA-inducible cytochrome P450 [P450RAI(CYP26)], in zebrafish, mouse, and human, which was shown to be responsible for RA catabolism. P450RAI exhibits a complex spatiotemporal pattern of expression during development and is highly inducible by exogenous RA treatment in certain tissues and cell lines. Sequence analysis of the proximal upstream region of the P450RAI promoter revealed a high degree of conservation between zebrafish, mouse, and human. This region of the promoter contains a canonical retinoic acid response element (5'-AGT-TCA-(n)5-AGTTCA-3'), embedded within a 32-bp region (designated R1), which is conserved among all three species. Electrophoretic mobility shift assays using this element demonstrated the specific binding of murine retinoic acid receptor-gamma (RARgamma) and retinoid X receptor-alpha (RXRalpha) proteins. Transient transfection experiments with the mouse P450RAI promoter fused to a luciferase reporter gene showed transcriptional activation in the presence of RA in HeLa, Cos-1, and F9 wild-type cells. This activation, as well as basal promoter activity, was abolished upon mutation of the RARE. Deletion and mutational analyses of the P450RAI promoter, as well as DNase I footprinting studies, revealed potential binding sites for several other proteins in conserved regions of the promoter. Also, two conserved 5'-TAAT-3' sequences flanking the RARE were investigated for their potential importance in P450RAI promoter activity. Moreover, these studies revealed an essential requirement for a G-rich element (designated GGRE), located just upstream of the RARE, for RA inducibility. This element was demonstrated to form complexes with Sp1 and Sp3 using nuclear extracts from either murine F9 or P19 cells. Together, these results indicate that the P450RAI-RARE is atypical in that conserved flanking sequences may play a very important role in regulating RA inducibility and expression of P450RAI(CYP26).

Differential expression of the retinoic acid-metabolizing enzymes CYP26A1 and CYP26B1 during murine organogenesis

We recently cloned the murine homologue of Cyp26B1, a novel retinoic acid (RA)-metabolizing enzyme and showed that its gene expression pattern is unique from that of Cyp26A1 during early embryogenesis. Here, we complete this comparative expression analysis from embryonic day (E) 12 to postnatal stages. Cyp26B1 expression was found in developing tendons and precartilaginous elements and in perichondrium by E14.5, while Cyp26A1 expression was restricted to extremities of rib and vertebral cartilage. Cyp26A1 and Cyp26B1 were expressed, in the distal epithelium and mesenchyme of the limbs and genital tubercle, respectively. High Cyp26B1 expression was found in craniofacial areas undergoing morphogenetic growth, whereas Cyp26A1 message was restricted to the mouth and dental epithelium. Cyp26A1 alone was expressed in the developing neural retina, while both genes were co-expressed in the retinal pigment epithelium. Cyp26B1 was specifically expressed in the developing hindbrain (pons, cerebellum) and forebrain (striatum, hippocampus), with forebrain expression persisting postnatally. In addition, Cyp26B1 was expressed at specific levels of the differentiating upper and lower thoracic spinal cord, adjacent to the cervical and lumbar regions that express the RA-synthesizing enzyme RALDH-2. In viscera, Cyp26B1 transcripts were detected in the developing lung, kidney, spleen, thymus and testis, whereas Cyp26A1 transcripts were found in the diaphragm and outer stomach mesenchyme. Cyp26B1 was also specifically expressed in dermis surrounding the developing hair follicles. Regulated RA metabolism may therefore be required in many developing systems.

Mouse P450RAI (CYP26) expression and retinoic acid-inducible retinoic acid metabolism in F9 cells are regulated by retinoic acid receptor gamma and retinoid X receptor alpha

We have cloned a mouse cDNA homolog of P450RAI, a cytochrome P450 belonging to a new family (CYP26), which has previously been isolated from zebrafish and human cDNAs and found to encode a retinoic acid-inducible retinoic acid hydroxylase activity. The cross-species conservation of the amino acid sequence is high, particularly between the mouse and the human enzymes, in which it is over 90%. Like its human and zibrafish counterparts, the mouse P450RAI cDNA catalyzes metabolism of retinoic acid into 4-OH-retinoic acid, 4-oxo-retinoic acid, 18-OH-retinoic acid, and unidentified water-soluble metabolites when transfected into COS-1 cells. Retinoic acid-inducible retinoic acid metabolism has previously been observed in F9 murine embryonal carcinoma cells and some derivatives lacking retinoid receptors. We were interested in determining whether P450RAI could be responsible for retinoic acid metabolism in F9 cells and in studying the effect of retinoid receptor ablation on P450RAI expression. In wild-type F9 cells and derivatives lacking RAR gamma, RAR alpha, and/or RXR alpha, we observed a direct relationship between the level of retinoic acid metabolic activity and retinoic acid-induced P450RAI mRNA. These experiments, as well as others using synthetic receptor subtype-specific retinoids, suggest that the RAR gamma and RXR alpha receptors mediate the effects of retinoic acid on the expression of the P450RAI gene.

CYP2U1, a novel human thymus- and brain-specific cytochrome P450, catalyzes omega- and (omega-1)-hydroxylation of fatty acids

Long chain fatty acids have recently emerged as critical signaling molecules in neuronal, cardiovascular, and renal processes, yet little is presently known about the precise mechanisms controlling their tissue distribution and bioactivation. We have identified a novel cytochrome P450, CYP2U1, which may play an important role in modulating the arachidonic acid signaling pathway. Northern blot and real-time PCR analysis demonstrated that CYP2U1 transcripts were most abundant in the thymus and the brain (cerebellum), indicating a specific physiological role for CYP2U1 in these tissues. Recombinant human CYP2U1 protein, expressed in baculovirus-infected Sf9 insect cells, was found to metabolize arachidonic acid exclusively to two region-specific products as determined by liquid chromatography-mass spectrometry. These metabolites were identified as 19- and 20-hydroxy-modified arachidonic acids by liquid chromatography-tandem mass spectrometry analysis. In addition to omega/omega-1 hydroxylation of arachidonic acid, CYP2U1 protein also catalyzed the hydroxylation of structurally related long chain fatty acid (docosahexaenoic acid) but not fatty acids such as lauric acid or linoleic acid. This is the first report of the cloning and functional expression of a new human member of P450 family 2, CYP2U1, which metabolizes long chain fatty acids. Based on the ability of CYP2U1 to generate bioactive eicosanoid derivatives, we postulate that CYP2U1 plays an important physiological role in fatty acid signaling processes in both cerebellum and thymus.

Cyp26C1 encodes a novel retinoic acid-metabolizing enzyme expressed in the hindbrain, inner ear, first branchial arch and tooth buds during murine development

Retinoic acid (RA), an active metabolite of vitamin A, is a crucial signaling molecule involved in tissue morphogenesis during embryonic development. RA distribution and concentration is precisely regulated during embryogenesis by balanced complementary activities of RA synthesizing (RALDH) and metabolizing (CYP26) enzymes. Here, we describe the identification of a novel murine p450 cytochrome belonging to the CYP26 family, mCYP26C1. Sequence alignment show that mCYP26C1 is more closely related to mCYP26B1 than mCYP26A1. At early developmental stages (E8.0-E8.5), mCyp26C1 is expressed in prospective rhombomeres 2 and 4, in the first branchial arch and along the lateral surface mesenchyme adjacent to the rostral hindbrain. At E9.5, mCyp26C1 expression persists in rhombomere 2 and in the maxillary and mandibular components of the first branchial arch, and is strongly induced in the lateral cervical mesenchyme. By mid-gestation, mCyp26C1 is weakly expressed in the cervical mesenchyme and in the maxillary component of the first branchial arch. At E11.5, mCyp26C1 can only be seen in a narrow band in the lateral cervical mesenchyme. During late gestation, mCyp26C1 exhibits region-specific expression in the inner ear epithelium and a persistent expression in the inner dental epithelium of the developing teeth. This pattern of expression suggests that mCYP26C1 may play an important role in protecting the hindbrain, first branchial arch, otocyst and tooth buds against RA exposure during embryonic development.

Identification of a novel retinoic acid receptor in regenerative tissues of the newt

In urodele amphibians, the progenitor cells that regenerate amputated limbs (known as the blastema) normally replace only the missing structures. After systemic delivery of retinoic acid (RA), more proximal structures are also formed, indicating that RA can control position specification in the proximal-distal axis of the regenerating limb. According to dose and experimental context, retinoids can also re-specify the anteroposterior axis of the limb, induce deletions of skeletal elements, or block re-growth completely. To study the molecular basis of these morphogenetic effects, we screened complementary DNA libraries of newt regenerative tissues (limbs and tails) for hormone nuclear receptors activated by RA. Two functional retinoic acid receptors (RARs) were identified, one of which is the newt homologue of the human alpha-receptor (RAR alpha). The second receptor, called RAR delta, is novel. Sequence analysis suggests that the composite newt RAR previously reported is chimaeric, consisting of 5'RAR-beta-like and 3' RAR delta clones. We conclude that multiple RARs are expressed during limb regeneration in amphibians and suggest that receptor heterogeneity may underlie the different effects of retinoids on limb morphogenesis.

The role of CYP26 enzymes in defining appropriate retinoic acid exposure during embryogenesis

Retinoic acid (RA) is a pleiotropic derivative of vitamin A, or retinol, which is responsible for all of the bioactivity associated with this vitamin. The teratogenic influences of vitamin A deficiency and excess RA in rodents were first observed more than 50 years ago. Efforts over the last 15-20 years have refined these observations by defining the molecular mechanisms that control RA availability and signaling during murine embryonic development. This review will discuss our current understanding of the role of RA in teratogenesis, with specific emphasis on the essential function of the RA catabolic CYP26 enzymes in preventing teratogenic consequences caused by uncontrolled distribution of RA. Particular focus will be paid to the RA-sensitive tissues of the caudal and cranial regions, the limb, and the testis, and how genetic mutation of factors controlling RA distribution have revealed important roles for RA during embryogenesis.

Transcriptional co-operativity between distant retinoic acid response elements in regulation of Cyp26A1 inducibility

Cyp26A1 encodes an RA (retinoic acid)-catabolizing CYP (cytochrome P450) protein that plays a critical role in regulating RA distribution in vivo. Cyp26A1 expression is inducible by RA, and the locus has previously been shown to contain a RARE (RA response element), R1, within the minimal promoter [Loudig, Babichuk, White, Abu-Abed, Mueller and Petkovich (2000) Mol. Endocrinol. 14, 1483-1497]. In the present study, we report the identification of a second functional RARE (R2) located 2.0 kb upstream of the Cyp26A1 transcriptional start site. Constructs containing murine sequences encompassing both R1 and R2 showed that these elements work together to generate higher transcriptional activity upon treatment with RA than those containing R1 alone. Inclusion of R2 also dramatically enhanced the sensitivity of reporter constructs to RA, as even treatment with 10(-8) M RA resulted in a 5-fold induction of reporter activity. Mutational analysis identified R2 as the functional element responsible for the increased RA inducibility of promoter constructs. The element was shown to bind RARgamma (RA receptor gamma)/RXRalpha (retinoid X receptor alpha) heterodimers in vitro, and inclusion of nuclear receptors in transfections boosted the transcriptional response. A construct containing both R1 and R2 was used to generate a stable luciferase reporter cell line that can be used as a tool to identify factors regulating Cyp26A1 expression. The analysis of R1 and R2 has led to the proposal that the two elements work synergistically to provide a maximal response to RA and that R2 is an upstream enhancer.

A zebrafish retinoic acid receptor expressed in the regenerating caudal fin

Retinoic acid (RA) is an important signalling molecule in vertebrate pattern formation both in developing and regenerating tissues. The effects of RA are due largely to regulation of gene transcription, mediated by retinoic acid receptors (RAR-alpha, RAR-beta, RAR-gamma) and retinoid X receptors (RXR-alpha, RXR-beta, RXR-gamma). We have been using zebrafish as a model of regeneration to study the role of retinoic acid and its receptors in vertebrate pattern formation. In this report, we describe the molecular cloning and characterization of one of the zebrafish RARs that is the predominant receptor in the regenerating caudal fin and corresponds most closely to the RAR-gamma subtype isolated from mouse and human and to RAR-delta from newt. Zebrafish RAR-gamma (zfRAR-gamma) exhibits both structural and functional conservation with its mammalian counterparts. Studies utilizing both normal and regenerating caudal fins of the zebrafish have indicated that it is the RAR-gamma subtype, compared to RAR-alpha or RAR-beta, which is expressed at the highest levels in the tail fin. To localize the expression pattern of RAR-gamma during fin regeneration, we have carried out whole-mount in situ hybridization. ZfRAR-gamma transcripts, during fin regeneration, are localized in the blastemal tissue formed at the distal ends of the bony rays following amputation. Treatment of fish with RA during fin regeneration induces a number of striking morphological effects on the regenerate. When amputations are performed distal to the branch points or dichotomies, where a single ray bifurcates to extend two individual ‘daughter’ rays, RA treatment causes a dichotomy reduction where the two ‘daughter’ rays fuse to once again form a single ray. The single ray subsequently bifurcates in a comparatively normal manner. Our data suggest that exogenous RA can respecify pattern in the regenerating caudal fin and identifies the blastemae as possible RA target tissues.

Regulation of gene expression by vitamin A: the role of nuclear retinoic acid receptors

Over the past five years, a wealth of information has accumulated concerning the molecular mechanisms mediating RA effects on gene expression. The molecular cloning of the 3 retinoic acid receptors (RARs), of their 16 or so different isoforms, and of the 3 retinoid X receptors (RXRs) as well as the identification of at least 2 different active isomers of RA (all trans- and 9-cis-RA) and of several different CRABPs and CRBPs now provide the essential tools to explain the pleiotropy that has become associated with RA effects. In the years to come, a concentrated effort to delineate the complex interactions between the various components of the retinoid signal transduction system should shed light on the mechanisms underlying pattern formation during vertebrate development and point to new ways in which retinoids can be exploited therapeutically.